This application claims the benefit of co-pending German Patent Application No. 100 55 405 entitled xe2x80x9cMontageeinheit aus einem Bauteil und mindestens einer Schraubexe2x80x9d, filed on Nov. 9, 2000.
The present invention generally relates to an assembly unit including a component and at least one screw. More particularly, the present invention relates to an assembly unit including a bush which is located in a through hole in a way to be unrotatable. Generally, assembly technology increasingly requires assembly units in which a plurality of components forms a pre-assembled unit. The assembly unit including a component and usually a plurality of screws is then connected to a work piece. As a typical example for this technology, an upper element and a lower element have to be connected by one or more screws. The upper element as the component and the plurality of screws are combined to form a pre-assembled assembly unit to which the screws are captively connected. The assembly unit is then connected to the lower element by tightening the screws one after the other or simultaneously, especially with the use of a multi-spindle automatic screwing apparatus. Such assembly units are typically used in the production of automobiles. For example, the cylinder head cover or a cover of a transmission forms the component which is to be connected with the work piece by screws, in this case, the cylinder block and the housing of the transmission, respectively. Realizing such assembly units allows for a great level of pre-manufacture. The screws are already located at the component in a captive, aligned and pre-assembled way. During transportation, they are safe from being damaged, and they allow for fast connection with the work piece.
An assembly unit is known from International Application WO 95/21335. The known assembly unit includes a component, for example a cover of a housing, and at least one fixing element in the form of a screw. A third element is associated with each screw. This third element is a bush which is used to capture the screw. The bush is made of metal, especially of sheet metal. The bush increases the cost of manufacture of the known assembly unit. The bush is pressed into a respective through hole being located at the component which requires a separate pre-assembly step. The bush is fixed in the through hole of the component due to frictional engagement. The bush has a structural height which is a multiple of the structural height of the component in the region of the through hole. The screw itself also has a respective shape and a great axial length. A ring flange is located in the region of the shank of the screw, the ring flange including ring channels at both sides. The ring flange has to be produced at the shank of the screw in addition to the thread. The size of the ring flange is coordinated with the size of a collar being located at the bush in a way that an axial lock is formed, the axial lock still allowing for a respectively great axial movement in a certain region of movement in a way that the screws in the pre-assembled position are located to be captive, but not fixed at the component. There also is the possibility of not arranging a ring flange at the screw. In this case, special elements have to be arranged at the bush, the elements in combination with the screw forming an axial lock. Consequently, the bush gets a more complicated design. In this case, the thread of the screw may fulfill part of the axial lock. There only is a relation between the diameter of the through hole being located in the component and the diameter of the shank of the screw in a way that the screw together with the bush pass through the through hole and have to be inserted into the through hole, respectively. However, axial alignment of the screw at the component does not take place since the screw is arranged to be only movable to a limited extent. This may be advantageous for screwing the screw into the work piece if it is ensured that the screw finds the entrance into the thread being located into the work piece in a centered way. Due to the possibility of tilting the excess of the screws with respect to one another entrance of the free end of the shank into the thread in the work piece may only be realized to some extent. These disadvantages may be counteracted by the bush as third component within the assembly unit having a relatively great axial length. Especially, it may be designed to be substantially longer than the thickness of the component. Due to the length of the bush, the length of the screw also has to be increased. As a result, not only costs are increased, but in some cases there is a negative effect on the function. Using such bushes in assembly units including at least three single elements is disadvantageous since the bush is subjected to tension while the screw is being screwed into the work piece. In case the bushes are made of comparatively thin deformable sheet metal, there is the danger of different deformations of the bushes occurring during tightening of the screws in a way that it is only possible to apply reproducible pre-tensioning forces to a limited extent.
German Patent Application No. 196 51 838 A1 teaches a component being made of plastic material which is connected to a work piece by screws with an intermediate arrangement of a seal. A bush protruding through the component and being connected to the seal is used. The screw has to include a separately produced ring flange which cooperates with the bush in the sense of an axial lock. In this way, the screw is arranged in a lose, but captive way at the bush. The bush is a third element of the assembly unit in addition to the component and the screw. The bush does not necessarily have to extend through the entire through hole in the component.
German Patent No. 195 46 072 C2 shows a bush having a shorter axial length and including a conical collar only partly engaging a partially conical through hole in the component under frictional engagement. The captive arrangement is realized by frictional forces in a way that the construction is respectively unsafe. The bush being designed in the form of a disc finally is located between the supporting surface at the head of the screw and the respective counter surface at the component. In this way, the bush is clamped, and there generally are similar disadvantages, as this has been described above.
Some known assembly units use screws having a trilobulare cross section in the region of the shank and which produce a thread. This allows for the advantage of not having to use the bush as a third element. On the other hand, there is the danger of chips occurring when the screws are screwed into the through holes in the component. Such chips are not acceptable in combination with special work pieces, for example with transmissions of automobiles. Furthermore, the known screw having a trilobulare cross section has the disadvantage of the carrying portion of the thread not being constant over the circumference. Changing screwing moments occur during assembly.
The present invention generally relates to an assembly unit. More particularly, the present invention relates to an assembly unit including a component, one ore more screws and the same number of bushes. The screw includes a shank at least partially including a thread, a head being designed and arranged to rotate the screw and a supporting surface facing the component. The supporting surface is designed and arranged to directly contact the component after assembly without contacting the bush to transmit an axial force. The component includes a through hole for each screw. The bush is associated with the screw, and it is designed and arranged to be fixedly connected in the respective through hole. The bush, the screw and the component are designed and arranged to be locked and aligned in an axial direction. This is achieved by the diameter of the through hole, the outer diameter and the inner diameter of the bush and the shank of the screw being designed and arranged with respect to one another in a way that the screw after its pre-assembly protrudes through the through hole in a fixed manner.
The novel assembly unit is also suitable for screws which do not produce a thread, for example for screws having a metric thread. The screws are securely connected to the pre-assembled assembly unit, and their axes are especially located in a perpendicular direction. Furthermore, the screws are captively connected to the component in the pre-assembled position. To be captively connected is to be understood as being connected in away that the screw is detachable from the component, but it cannot get lost unintentionally.
The novel assembly unit includes one or more screws which may also have a metric thread. However, it is also possible to use screws which form or produce a thread, preferably screws which produce a thread during screwing without producing chips. There is a certain relation between the diameter of the through hole being located in the component, a bush which is to be fixedly arranged in the through hole and the size of the screw, especially in the region of its shank including a thread. In this way, the screw extends through and engages the through hole, respectively, in a fixed manner after pre-assembly. Thus, not only an axial lock is realized, but also a determined alignment of the axis of the screw with respect to the component. The alignment of the axis of the screw is determined in this way, and the screw cannot tilt. In case the screw only partly protrudes into the through hole, the thread of the screw is partly located in the region of the bush and partly at the side of the screw facing the head outside the through hole. In this way, the thread is located in a safe position during transportation from the manufacturer of the pre-assembled assembly unit to the place where the assembly unit is further used such that there is no damage. The screw or the screws are located in a fixed position and with aligned axes in the respective bush (and these in the through holes). As a result, function is improved and a captive arrangement is realized.
The bush forms a plug-like pre-assembly element for screwing in the screw. The supporting surface being located at the screw and serving to transmit the axial force is designed and arranged to directly contact the component without the bush. There is no negative effect on the application and the effect of the axial force in the tightened position of the screw. Preferably, the bush is made of plastic material, and it may be deformed without producing chips when the thread of the screw is screwed into it. The bush in the tightened position of the screw remains free from any axial force such that disadvantageous setting effects under a decrease of free tension are not to be expected. To attain a sealing effect between the work piece, the component and the head of the screw, it may make sense to chose the size of the bush such that a limited axial force required to attain the sealing effect is transmitted by the bush. These axial forces remain comparatively small such that no setting effects are to be expected in this case.
It may make sense that the screw in the region of its shank close to the supporting surface includes a ring groove which forms an undercut. The ring groove has a diameter which is less than the inner diameter of the bush and an effective axial length which is more than the length of the bush. Especially when a plurality of such screws is to be located in the pre-assembled position of the component in a way that it is designed for multiple screw connections with the work piece, it is important that the screw in the region of the shank between the supporting surface at the head and the threaded portion includes a ring groove forming an undercut. The diameter of the ring groove is less than the inner diameter of the bush, and its axial length is more than the axial length of the through hole in the component taking the respective transition radiuses into account. Due to the fixed arrangement of the screw, the axial lock and the alignment of the axis as well as the captive arrangement of the mounted assembly unit are reached in the pre-assembled unit. On the other hand, the screw and the screws, respectively, in the region of the ring channel forming the undercut gets free from the bush being located in the component in a radial and in an axial direction such that there is an adaptation of the screw or the screws at the work piece taking tolerances into account. As soon as the screw with its thread has exited the elastic-plastic deformable thread in the bush in the through hole of the component, the component may move and adapt, respectively, to a limited extent in an axial and/or a radial direction with respect to the work piece. The pretension force is applied by further tightening the screw. When a plurality of screws is used, a possible clamping effect of the component at the work piece due to the screws is eliminated when the threads of the screws exit the bushes. Consequently, the component reaches a free relative position with respect to the work piece. This adaptation may also be used when the screws are screwed into the work piece one after the other. The same applies when a plurality or all screws are screwed into the work piece simultaneously. In this case, it is especially advantageous to use screws which produce a thread, and not to arrange threads in the bores in the work piece which are engaged by the screws. Instead, the bores are designed as cast or cut bores. Then, it is not necessary to produce threads in the work piece during its manufacture. Instead, the screw produces the thread. Mutual clamping forces subjecting the component as a result of a plurality of screws are prevented since each screw produces its own thread in the bore in the work piece during its insertion into the bore. The relative angle position of the threads of the screws during the screwing in process with respect to one another are automatically taken into account. When respective tolerances are met, it is also possible to connect the pre-assembled assembly unit including the component and the screws with a work piece the bores of which are associated with the screws already including threads. This applies no matter whether a plurality of screws is simultaneously screwed into the work piece or one after the other. The use of the bush being made of a resilient material, especially of plastic, is advantageous.
The thread of each screw at its side facing away from the head of the screw may include a connecting portion in which the outer diameter of the thread is less than the inner diameter of the bush being located in the through hole in the component. This coordination is chosen in a way that the screw with the connecting portion may be inserted into the bush and, on the other hand, the function of the thread of the screw to produce a thread is used for an at least elastic deformation of the bush when the screw is screwed into the bush.
In all cases, the shank of the screw in the region of the thread may have a trilobulare cross section. Such a trilobulare cross section realizes a so-called orbiform. This means that there is the same size at all places around the circumference of the cross section when the diameter is determined with a change of 180 degrees, although the cross section approximately has a triangular shape. Such a trilobulare cross section also fulfills the function of a self-centering effect, and it allows for elastically/plastically forming the thread in the bush and possibly also in the work piece.
The bush may be fixedly arranged in the through hole of the component due to frictional engagement and/or to form fit. In this way, the bush is held in the through hole of the component in a way that it cannot be rotated. The through hole in the component and/or the outer diameter of the bush may have an unround design for this purpose. For example, it may be designed as a polygon, to include a radial protrusion, to have an oval cross section, as a square or the like. The fixed connection due to frictional engagement is realized by a respective size and by pressing the bush into the through hole. This may be achieved in different ways. The bush may be fixedly arranged in the through hole with an excess or surplus excess at both sides, at one side or without an excess.
The bush being located in the through hole of the component may have an inner diameter which is more than the core diameter of the shank of the screw which includes the thread. The design of a thread in the bush is less important since a screw with its thread gets free from the bush during tightening, anyway. It is sufficient if the screw in the pre-assembled position takes its fixed, aligned and captive seat in the bush which acts like a plug.
To prevent clamping of material of the bush during tightening, the screw may include an impression being designed as a continuous yielding channel which is arranged close to the supporting surface. The yielding channel is necessary in case the bush has an axial and radial excess towards the outside in the position after it has been pressed into the through hole. Usually, the yielding channel will be arranged inside at a smaller diameter, and the supporting surface will be arranged outside at a greater diameter.
The bush may have a greater structural height than the component in the region of the through hole. The bush at its side facing the head of the screw may include a collar protruding beyond the through hole in a radial direction or respective protrusions which limit the capability of pressing the bush into the through hole of the component. The collar and the protrusions form a stop when the screw is screwed in, and they prevent the bush from being pushed out off the through hole. Even without a collar and protrusions, the bush at least at its side facing the head of the screw outside the height of the component in the region of the through hole may be designed to be expandable by screwing in the screw. Consequently, the excess of the bush extending in an outward direction forms a stop protruding in a radial direction when the screw is screwed in.
The bush at its side facing away from the head of the screw may alternatively or additionally include a protrusion which protrudes over the through hole in the component in an axial direction. The protrusion forms an expanding rip to ensure fixed connection of the screw at the pre-assembled unit.
The screw at the free end of its shank may include a centering connection or a centering element having no thread. The centering connection has a smaller outer diameter than the inner diameter of the bush. The centering connection makes sense when the screw is screwed into the bush and into the threaded hole or the thread in the work piece.
The bush may have an inner diameter which is more than the flank diameter and which is less than approximately 90 percent of the outer diameter of the thread being located at the shank of the screw. The diameter of the through hole is located in the region defined by these two limits. In case these limits are kept, resistance is sufficiently small when the screw is screwed into the bush. The necessary pretension force may be applied after screwing in the screw in the desired and a reproducible way. Due to a special design of the collar being located close to the head of the bush, the bush may be pressed during tightening of the screw in a way that it fulfills the function of a sealing element being located below the head of the screw. For this reason, the size of the collar of the bush is coordinated with the size of the recess in the head of the screw. Again, no plastic material of the bush is pressed in when the screw is tightened. Thus, there is no danger of undesired setting effects and pretension losses occurring in the novel screw connection.
Other features and advantages of the present invention will become apparent to one with skill in the art upon examination of the following drawings and the detailed description. It is intended that all such additional features and advantages be included herein within the scope of the present invention, as defined by the claims.